In the industry of composite material, for structural and cosmetic products having dimensions of some importance, the projection of fiber glass followed up by lamination is a process which has been used for quite sometime. It has always occupied an important part of small lot market due to the low cost of molds which is balanced off by the high cost of manpower. In the 90's, the robotization of this process has enable to accelerate the production cycle, to reduce thickness variations and, consequently, to considerably increase the profitability of higher volume lots.
In general, the process consist in projecting a coating of unsaturated polyester or in thermoforming an acrylic sheet on an open mold. Thereafter, chopped glass fibers are added simultaneously with a catalyzed liquid resin by either manual or robotized projection. Thereafter, lamination is carried out and air bubbles or voids are removed with rollers and brushes. The resin then solidifies, the work piece is demolded and machine-finished, and trimming completes the making of the product.
A cutter roller (or chopper) serves to cut (more exactly break) from one to four continuous strands of glass fibers to a useful length of about 0.5" to 2" according to the mechanical requisites of the product. The cut fibers are simultaneously projected on a flat and elliptical stream of resin, which is projected under high pressure through a minuscule gun orifice to confer to the spray a high kinetic energy which carries the cut fibers towards the mold. The projections of glass fibers and resin are adjusted in order to obtain a ratio adequate to provide the desired mechanical properties of the product to be molded.
The cutter roller is generally pneumatically driven (it is sometimes hydraulic) as it is used in a medium of volatile materials that are inflammable and potentially explosive. Furthermore, this tool has been historically, and is still being used, manually or mounted on paint robots having a low payload. It has therefore been required to limit the weight of this equipment.
The glass fibers are directed on only one side of the resin stream where they are wetted and brought to the mold. With glass/volume ratios of 12%, and more, an important part of the cut fibers do not penetrate the resin stream and slide therealong. Hence, depending on the traveling direction of the cutter and the resin gun, a rich mixture of resin is obtained on one side but a poor mixture is present on the other side; hence, when lamination is then carried out on the mixture, this difference creates conformation variations and high risk of air bubbles or voids while contributing to varying thicknesses. One must therefore allow more time to carry the lamination/debubbling steps of the mixture than to project it. Furthermore, the presence of areas which are resin rich near the coating favours the creation of cracks under stress conditions occurring during the use and operation of the molded product.
There are two methods of applying such material to an open mold. One named "deposition in an airless environment" consists of a robotized projection carried out under vacuum conditions of cut glass fiber roving and a catalyzed resin. This process enables to eliminate micro- bubbles which expand under furnace heat. It also provides for an important reduction in manpower, in lamination and in finishes and repairs. An immediate conformation of the fibrous mixture is achieved wherever a radius of curvature in the mold enables an impact at about 90.degree.. The smaller radii of curvature are laminated since fibers having a length of about 1" to 11/4" form bridges over these radii. The conformation by lamination must be facilitated by the absence of air trapped under these compact bridges.
A second method is used for fiberglass products which are cosmetic as well as structural; the process requires production cycles which are relatively high and operates at ambient temperature. The products obtained by this process are those having a colored gel coat finish. Hence, for these products, the micro air bubbles (less than 1 mm) which are formed during atomization under atmospheric conditions have no effect.
This method consists also of a robotized projection of cut fiberglass roving and of a catalyzed resin but carried out under atmospheric condition. In projecting multiple fine layers, an immediate conformation of the fibers mixture is obtained everywhere where the radius of curvature of the mold enables an impact at about 90.degree.. The smaller radii are laminated since fibers having a length of 1 " to 11/4" form bridges over these radii.
The air bubbles (smaller than 1/4") which may remain are almost always located in radii which are highly structural and not much subject to impact during use. Hence, there is previously added between the gel coat and the fibrous substrate a flexible barrier layer which is resistant to impact. When this barrier is added in sufficient thickness, the radius of curvature is reduced thereby facilitating the conformation of the spray and providing protection against bubble puncturing. The barrier layer also improves the cosmetic aspect of the external finish (gel coat).
The above two methods greatly reduces lamination, which enables to better control the thickness of the sprayed material.